SHORT NOTES AN ESTIMATE OF THE NUMBERS OF SHEARWATERS ON THE NECK, SKOMER, 1978 M. Alexander and C.M. Perrins Summary An estimate was made of the number of fledgling Manx Shearwaters, Puffinus puffinus, on the Neck, Skomer, in 1978. The estimate is compared with one made in 1971. The result suggests that numbers have probably remained stable, or perhaps even increased slightly, during the period concerned. INTRODUCTION Skomer Island, a National Nature Reserve, houses a very large breeding population of Manx Shearwaters, Puffinus puffinus. Burrows containing breeding birds are widely dispersed over the whole island. Partly because the birds are nocturnal, it has proved exceptionally difficult to estimate the number of pairs involved or guess whether the numbers are stable, increasing or decreasing. The only previous quantitative attempt to estimate the numbers of Manx Shearwaters on Skomer was made in 1971 by Peter Corkhill. He estimated the number of fledgling Shearwaters which were raised on one part of Skomer, the Neck. Using other measures of breeding success, he related the number of fledglings to the number of breeding pairs. Finally, he compared the size of the areas used by Shearwaters on the rest of Skomer with that of the Neck and arrived at a rough estimate of 95,000 breeding pairs of Shearwaters for the whole island. In order to have some idea of the status of the Shearwater on Skomer, we decided to repeat Corkhill's census of the Neck in 1978. METHODS Corkhill (1973) estimated the number of fledglings using the method described by Perrins (1967) for Skokholm. This is essentially a "mark, release and recapture" technique which involves (1) the marking of a sample of young Shearwaters in their burrows, (2) the capture of a largish number of young birds on the surface at fledgling ti ne, and (3) the calculation of the total using the following formula: